1. Technical Field
The present invention relates to a copier, a printer, a facsimile machine and an image forming apparatus using an electrophotographic method having the functions of the copier, the printer and facsimile machine. Particularly, the present invention relates to a color image forming apparatus including an intermediate transfer member for superimposing plural color toner images onto the intermediate transfer member to form an image.
2. Description of the Related Art
In the electrophotographic method color image forming apparatus using the intermediate transfer member, known is an image forming apparatus arranged to transfer a toner image formed on an image carrier, which is a photoreceptor onto the intermediate transfer member (primary transfer), then the toner image on the intermediate transfer member is collectively transferred onto a transfer material (secondary transfer). In this type of color image forming apparatus, the color image forming apparatus is designed to superimpose electrostatic toner images, which has been sequentially formed on the image carrier with a predetermined polarity, onto the intermediate transfer member by using static electricity. Then the toner images on the intermediate transfer member are collectively transferred onto the transfer material electrostatically.
Since an electrostatic charge amount per a toner particle is approximately uniform, the electric potential of the toner layer on the intermediate transfer member is determined by the toner adhesion amount in a predetermined area. In the color image forming apparatus, the electrostatic potential of the area where toners of plural colors are superimposed among the toner images of the intermediate transfer member becomes higher than that of the area where one color toner adheres. And for example, when there are a toner image of a solid area and a toner image of a halftone area on the intermediate transfer member, the electrostatic potential of the solid area is higher than that of halftone area.
Dispersion of the electrostatic potential in the image area having passed the primary transfer unit which transfers the toner image onto the intermediate transfer member from the image carrier may be generated according to environments.
As described above, when toner image potential dispersion on the intermediate transfer member is large, areas where transfer characteristics are different with each other exist in the same toner image. When transferring all the areas where the transfer characteristics are different with each other onto the transfer material under the same transfer condition, various poor quality images tend to appear when transferring the toner images from the intermediate transfer member onto the transfer member.
In recent years, in the image forming apparatuses such as the copier, the printer, the facsimile machine and multifunctional peripherals having the function thereof, the ratio of the apparatuses having color capability has increased. At the same time, along with the adoption of polymerization toner and toner having a small diameter, the requirements for high quality images in a transfer process has increased. Further, a high-speed process trend improves in the image forming apparatus. In response to these trends described above, in order to obtain a high quality image, it is necessary to correct the toner potentials on the intermediate transfer member, which vary according to the number of times of the primary transfer and the environment, so as to be approximately uniform, and to improve the second transfer performance.
In the color image forming apparatus for conducting the secondary transfer of a toner image from the intermediate transfer member to the transfer member after superimposing the toner image of each color formed on the surface of a photoreceptor onto the intermediate transfer member by using the primary transfer unit, since the charge amount of the toner on the intermediate transfer member varies according to the number of times of the primary transfer and environments, various image failures tend to be caused.
In the electrophotographic recording apparatus disclosed in Unexamined Japanese Patent Application Publication No. H08-202171 (JPA8-202171), provided is a determination means which determines the contamination in the scorotron based on the electric current amount flowing to the charging wire of the scorotron which being a pre-transfer charging means, the electric current amount flowing to the shield member, and the electric current amount flowing to the grid electrode; and a cleaning means which cleans the charging wire in the scorotron based on the determination of the determination means.
The charging apparatus disclosed in Unexamined Japanese Patent Application Publication No. H09-297457 (JPA9-297457) is provided with a grid cleaning means which cleans the grid by pressing it while being moved by a driving mechanism.
The electrophotographic recording apparatus disclosed in JPA8-202171 is a charging apparatus for charging a photoreceptor with certain amount of discharge, which is able to calculate the discharging amount onto the photoreceptor based on each current value flowing into a charging wire, a shield member and a grid electrode. However, with the charging apparatus, it is difficult to determine the variation of the discharging amount is caused by the dirt of which part of the charging apparatus, among the charging wire, the shield member and the grid electrode. Herein, a cleaning means is for cleaning the charging wire. Since it is difficult to maintain the difference between the electric potential of a toner image after the pre-secondary transfer discharge and the electric potential of the intermediate transfer member substantially constant, it is impossible to stably transfer the toner image on the intermediate transfer member onto a transfer material, which causes deterioration of the toner image quality formed on the transfer material.
In the charging apparatus described in JPA9-29745, cleaning of the grid is conducted, but since the cleaning is periodical, it is conducted irrelevantly to the actual dirt (contamination) of the grid. Therefore, even when frequent image failures are generated, responding action is not taken. Further, if the frequency of the cleaning is increased, the productivity will be decreased and the durability of the grid will be lowered.
An object of the present invention is to provide an image forming apparatus for preventing the degradation of electric potential control ability generated by the dirt on the grid electrode and the image roughening in the halftone area, improving the durability of the grid electrode, and for achieving the improved secondary transfer ability to obtain a high quality secondary transfer image, by correctly detecting the dirt on a grid electrode generated by the adhesion of floating toner in a pre-secondary-transfer discharge unit.